The concept of habit plays a central role in Thomas Aquinas’ moral theory, and in his analytic psychology more generally. He identifies habits as one of the fundamental principles of human action, together with the capacities of intellect, passions, and will – appropriately so, because habits are nothing other than stable dispositions of these capacities, which enable them to operate in coherent, appropriate ways. Habits are subject to moral evaluation, just as actions are, and the habits of the passions and the will are always either virtues or vices, just as human actions are always morally good or evil. Even more fundamentally, habits of some kind are necessary to the functioning of both the appetites and the intellect. Without some kind of internal development and formation, the appetites and the rational powers of the human creature cannot function at all, or can only operate in rudimentary and ineffective ways. By implication, human action as we know it, whether from our experience as agents or by observation of others, is almost always shaped by habits of some kind, formed out of natural (or supernatural) principles of operation through processes of training and development. Human existence and action is ultimately grounded in natural principles, but the innate principles of human action do not enter directly into our ordinary experiences – rather, we act and interact with one another through the structures set up by the stable dispositions of our virtues, vices, and other habits.
These are claims about the origins and structuring principles of human action, and as such, they invite comparison with other kinds of claims about human psychology, including those set forth by contemporary experimental psychology. A comparison of this kind might seem to be ruled out by the radical differences in assumptions and methodology that divide Aquinas from contemporary scientists of any kind. Yet when we compare what Aquinas says about the habits, we find unexpected resonances with recent work on the formation of stable dispositions, especially those relating to such fundamental matters as food preferences. We need to be careful not to overstate the extent and significance of these resonances. However, it would seem that at the very least, Aquinas and contemporary psychologists share points of reference that enable us to compare them in fruitful ways. To put it crudely, we can assume that they are talking about the same things, more or less, namely, human activities and experiences, together with whatever principles or structures account for these.
I want to defend and develop this way of approaching Aquinas’ psychology by comparing what he says about the formation and necessity of habit with recent work on the formation of food preferences in infants. This research lends support to Aquinas’ view that even the most fundamental human capacities for desire and aversion need some kind of rational formation and structuring in order to function properly. Indeed, Aquinas’ analytical psychology and contemporary experimental psychology seem to converge, and for a Thomist, this convergence suggests that research in this area might tend to confirm—or at least shed light on—Aquinas’ account of a virtue such as temperance, as it pertains to the pleasures of food and drink. When we turn back to relevant studies, however, it would seem that they challenge Aquinas’ account of temperance, insofar as he defines this virtue by reference to the individual’s bodily needs. On further reflection, contemporary work on food preferences is not inconsistent with Aquinas’ account of temperance, but it does suggest that a Thomistic account of the virtue of temperance needs to be expanded and developed in certain ways, if we are to do full justice to the complexities of developmental formation.

Habits are commonly associated with stereotyped, repetitive behaviors that often fall outside the range of one’s conscious awareness and may even be experienced as somehow contrary to one’s desires. A habit in the Thomistic sense more nearly resembles a skill, such as touch typing, than a habit like pulling one’s beard – indeed, the skill of touch typing is a habit in Aquinas’ sense, whereas beard-pulling is not. Habits understood in this way are clearly more interesting and important than the kind of habits that we only notice when we want to break them. But we have not yet taken full account of just how important habits are, on Aquinas’ view. He claims that habits of some kind are necessary for the proper functioning of the rational creature. Human capacities for perception, understanding, and desire are naturally indeterminate, and stand in need of some level of development in order to function at all.
Contemporary developmental psychology offers at least one example of a kind of system that would appear at first glance to function in much the same way as do the habits, as Aquinas understands them: “[E]ach of a multitude of core knowledge systems emerges early in development, serves to identify the entities in its domain by analysing their distinctive characteristics, and supports the acquisition of further knowledge about those entities by focusing on the critical features that distinguish different members of the domain (Shutts et al).” A system of core knowledge would thus seem to provide the same kinds of rational structures, and thus to facilitate appropriate functioning, in the same way that habits do. Systems of core knowledge would not be equivalent to habits as Aquinas understands them, but on the contrary, they would provide evidence that habits in this sense would be superfluous.

We can tentatively identify at least one such system that appears to be comprised of highly general concepts, which can only function properly after some level of formation, namely, the system of core knowledge with respect to food. For more than thirty years, psychologists have been studying the emergence and development of the infant’s ability to distinguish between edible and inedible objects, and between appropriate and healthy, or inappropriate, spoiled, or otherwise unsuitable food. In contrast to adults, older children, and at least some kinds of non-human primates, human infants are remarkably indiscriminate with respect to what they will ingest; one study summarizes that “items regarded by adults as dangerous, disgusting, or inappropriate, and combinations of individually liked foods that are unacceptable to adults were readily accepted by many of the younger children in our sample…. [However,] within the 16 month to five year age range studied, there is a clear developmental trend towards rejection of items that adults consider disgusting or dangerous.” (Rozin)

Indeed, recent research indicates that the food preferences of young children are not only formed through social interactions, but strongly tied to perceptions of, and feelings about, group identity. This research supports what experience would seem to confirm, that our habituated desires for specific foods may reflect our sense of social identity and a socially mediated sense of what is desirable, seemly, or appropriate for those of our kind. If this is so, then it would appear that at an early stage, the developing child begins to develop normatively laden food preferences, reflecting basic judgments about what one ought to eat.

This way of construing the development of food preferences is highly suggestive from the standpoint of a Thomist virtue ethicist. The virtue of temperance, for example, is a habitual disposition towards desires and aversions that reflect normative judgments of some kind. Any suggestion that early childhood food preferences develop in accordance with normative judgments will be grist to the mill for a contemporary Thomist. At the same time, we need to be careful not to move too quickly between contemporary psychology and Aquinas’ account of temperance. It is not immediately evident that the normative ideal informing temperance is the same as the normative standards informing the food preferences of young children. This does not mean that recent research disconfirms Aquinas’ analysis of temperance, but it does point to a more interesting way of thinking about the relation between these two very different ways of thinking about our most basic desires.

Kristin Shutts, Kirsten F. Condry, Laurie R. Santos, and Elizabeth S. Spelke, “Core Knowledge and its Limits: The Domain of Food,” Cognition. 2009 Jul; 112 (1): 120–140.

Paul Rozin, Larry Hammer, Harriet Oster, Talia Horowitz and Veronica Marmora, “The Child’s Conception of Food: Differentiation of Categories of Rejected Substances in the 16 Months to 5 Year Age Range,” Appetite, 1986, 7, 141-151.

Samantha P. Fan, Zoe Liberman, Boaz Keysar, Katherine D. Kinzler, “The Exposure Advantage: Early Exposure to a Multilingual Environment Promotes Effective Communication,” Psychological Science.  July 2015 vol. 26 no. 7 1090-1097.

Jean Porter is the John A. O’Brien Professor of Theological Ethics at the University of Notre Dame and Scholar with Virtue, Happiness, & the Meaning of Life.